We have continued our studies of the insulin-like growth factor/somatomedin, known as multiplication stimulating activity (MSA), the rat homologue of IGF-II, a polypeptide synthesized by a cloned line of rat liver cells (BRL-3A). During the past year, progress has been made in the following areas: (1) demonstration of a biosynthetic precursor of MSA (presumably pre-pro-MSA) in a cell-free translation system and of pro-MSA in intact metabolically-labeled BRL-3A cells; (2) demonstration by affinity crosslinking of two IGF receptor subtypes that differ in reactivity with insulin, IGF-I, and IGF-II, and in subunit structure; (3) confirmation of the proposed IGF-II receptor structure by receptor purification; (4) demonstration that insulin and MSA utilize insulin and IGF receptors, respectively, to mediate the same biological responses in hepatocytes, chondrocytes, myocytes, fibroblasts, and embryonal carcinoma cells; (5) demonstration that the type I IGF receptor of a rat liver cell line has intrinsic tyrosine kinase activity and autophosphorylates the Mr 98K Beta-subunit of the receptor; (6) demonstration that type I IGF receptors, but not type II receptors, are selectively downregulated in a mouse muscle cell line; (7) demonstration that insulin acutely increases the number of type II IGF receptors on the surface of intact rat adipocytes by redistribution of cycling receptors from a large intracellular pool; (8) demonstration that insulin desensitizes rats hepatoma cells to IGF-stimulated IGF-receptor mediated action at a step distal to hormone binding; (9) demonstration that IGF-II is the major somatomedin in fetal rat serum and that IGF-I levels increase shortly after birth; (10) demonstration that fetal/neonatal rat fibroblasts in culture synthesize IGF-II that is induced by ovine placental lactogen, whereas adult fibroblasts synthesize predominantly IGF-I induced by both growth hormone and placental lactogen; and (11) characterization of growth hormone-dependent binding proteins for somatomedins in human serum and fetal and postnatal rat serum.